Improved Impossible Differential Attacks against Round-Reduced LBlock

Impossible differential attacks are among the most powerful forms of cryptanalysis against block ciphers. We present in this paper an in-depth complexity analysis of these attacks. We show an unified way to mount such attacks and provide generic formulas for estimating their time and data complexities. LBlock is a well studied lightweight block cipher with respect to impossible differential attacks. While previous single-key cryptanalysis reached up to 22 rounds, by applying our method we are able to break 23 rounds with time complexity 2 and data complexity 2. Other time/data trade-offs are equally possible. This is to our knowledge the best (non-exhaustive search like) cryptanalysis of this function in the single-key model.